Field of the Invention
The disclosure relates generally to devices and methods of measuring gain of a voltage-controlled oscillator, and more particularly to methods and devices of in situ measuring of gain of a voltage-controlled oscillator of a phase-locked loop circuit.
Description of the Related Art
Many of today's electronic devices (e.g. cell phones, computers, personal digital assistants (PDAs), etc.) require high frequency clock signals in order to operate. Usually, these clock signals are generated using phase locked loops (PLLs).
A typical PLL includes a phase detector, a loop filter, a charge pump, and a voltage-controlled oscillator (VCO). While all of the components perform vital functions, it is really the VCO that is at the heart of the PLL because it is the VCO that provides the output high frequency signals, and it is the VCO that enables the frequencies of the output signals to be adjusted based upon a control voltage signal.
In designing a PLL, one of the starting points is the selection of an (output frequency)/(control voltage) value for the VCO. This gain, referred to herein after as KVCO, specifies how much the output frequency of the VCO will change given a change in control voltage. In practice, the KVCO will most likely vary with the control voltage and hence, would not be a constant. Nonetheless, to simplify the design process, a constant KVCO value is used as an approximation. After the KVCO value is selected, it is used to determine the parameters of the other PLL components. The KVCO value affects the performances of the PLL, such as the transfer function and frequency response of the PLL. In this manner, the PLL is designed basing on the VCO.
The KVCO value that is selected for a PLL depends upon the particular VCO that is to be used. This variety of KVCO values results not from any defect in the design of the VCO, but rather from the processing variations that are encountered in fabricating the VCO.
More specifically, a VCO includes a plurality of components (e.g. transistors). These components are manufactured by some fabrication process. Ideally, the fabrication process should produce identical components (components with the same parameters) each and every time. In practice, however, this is not possible. As a result, there will be some variation in the parameters of the components used to make up the VCO. Some of the components will have parameters that barely meet minimum specifications, while other components will meet or exceed the maximum specifications, while other components will fall somewhere in between. Because of these processing variations, the KVCO values exhibited by different VCOs will differ, even if the VCO's are all of the identical design. In addition, the reciprocal of the KVCO value is needed when designing a compensation filter for a PLL, which requires additional efforts due to the division operation. This means that we need an effective device and method to measure the reciprocal of the KVCO values of VCO in a PLL.